This invention relates to a band pass filter device and more particularly to a band pass filter device of the type having a plurality of thin metal stripes or slits formed on a surface of a plate made of a piezoelectric material or non-piezoelectric material for propagation of acoustic waves and operable in a frequency range of VHF or UHF.
A circuit device utilizing elastic surface waves such as a resonator or filter circuit device has been developed as a circuit used in the VHF or UHF frequency range in place of a conventional circuit utilizing a combination of concentrated circuit elements such as capacitance, resistance and inductance elements. This circuit has already been applied to a communication apparatus and a television receiver set. The surface wave circuit device has many advantages such as easy miniaturization, stable operating characteristic and realization of uniform characteristics at low cost but still has unsolved problems. Particularly, minimization of loss and attainment of desired frequency characteristic have not yet been reached.
In recent years, it has been desired to make a filter device, applicable to various communication apparatus and home appliances, which has a characteristic wherein loss is reduced in a pass band, attenuation is large in a stop band and rising at the boundary between the pass band and stop band is sharp. In other words, a square-wave-like frequency characteristic in the pass band is desired.
Various types of surface wave filter devices have hitherto been proposed to satisfy the above requirements, but existing filter devices are unsatisfactory to meet all of the requirements. U.S. Pat. No. 4,044,321 entitled "Surface acoustic wave band pass filtering" discloses a surface wave band pass filter device, similar to a filter device of the present invention, which can reduce spurious component due to bulk wave and produce large attenuation in a stop band.
The band pass filter device according to the U.S. Pat. No. 4,044,321 comprises two surface wave filters connected electrically in series having the same frequency characteristic f(.omega.) for surface waves but different frequency characteristics for bulk waves, whereby the total frequency characteristic is such that {f(.omega.)}.sup.2 is maintained in the pass band and an attenuation beyond {f(.omega.)}.sup.2 is obtained in the stop band.
However, the inventors of this application have found the above band pass filter device defective in that loss in the pass band is large and the frequency characteristic in the pass band is uneven while containing ripple.